Alkali metal chlorides, such as sodium chloride (NaCl) and potassium chloride (KCl), are commercially electrolyzed using cation exchange membranes to make chlorine and either sodium hydroxide (NaOH) or potassium hydroxide (KOH). The state-of-the-art process for such a chloralkali electrolysis is membrane electrolysis, in which a non-porous membrane separates the anode chamber and the cathode chamber. To minimize power consumption, it is important to maximize current efficiency and to minimize cell voltage. Membranes are commonly reinforced with a chemical-resistant fabric to improve tear strength, burst strength, and dimensional stability.
In order to obtain a low cell voltage in a chloralkali cell along with good stability for handling the reinforcing fabric and the reinforced membrane, it is desirable to have an open reinforcing fabric and a thin membrane. A thin membrane requires a thin fabric and a small total thickness of the film layers used in laminating the reinforced cation exchange resin.
An open fabric is one which, when illuminated from a direction perpendicular to the plane of the fabric, allows a large amount of the incident light to pass through the fabric. In other words, it is a fabric with a large percentage of open spaces. This is desirable because it is the open spaces which allow cations to pass from the anolyte to the catholyte in the chloralkali process. Thus, an open fabric makes possible a lower cell voltage and therefore a lower power consumption.
The simplest kind of fabric is one with a plain weave, shown in FIG. 1. If it is made with high openness--a small number of yarns in each direction--the fabric lacks dimensional stability and may stretch out of shape. For example, if a square piece is suspended from one corner, it will distort and no longer be square and flat. Stated another way, a membrane reinforced with such an open plain weave fabric is dimensionally unstable. This is a serious problem during assembly of commercial cells, particularly those which may require large membranes, some of which are as large as 1.5.times.3.7 meters (m), and those in which vertical assembly is employed.
In order to make a more open fabric with uniform open spaces than is feasible with a plain weave, considerable attention has been given to leno weave fabrics, shown in FIG. 2. For example, U.S. Pat. No. 4,072,793 teaches the use of leno weave fabrics. However, as can be seen from FIG. 2, the fabric tends to be thick at the point where two warp yarns cross a filling yarn at about the same place. Thick fabrics are generally considered undesirable because they require a large amount of polymer to cover the fabric on both sides of the membrane. If the yarn penetrates the surface of the membrane, it tends to cause leakage from one electrolyte to the other along voids that result because adhesion of the polymer to the yarn is imperfect. Leakage of catholyte into the anolyte causes low current efficiency and high power consumption along with other problems. Leakage of anolyte into the catholyte may lead to amounts of chloride in the caustic product which exceed customer requirements.
Although leno weaves provide an improvement over plain weaves, they too are biaxial structures that suffer from low modulus and strength in the bias direction.
The present invention solves this problem.
Multiaxial fabrics may be made by any of the technologies as described in Scardino, Chemiefasern/Textilindustrie, Vol. 35/87 (April, 1985), pp. 268-271. One is multiaxial stitchthrough technology, another is multiaxial non-woven technology, and a third is triaxial woven technology. Others include braided, interstitched and interknitted. Yarns are generally held together by interlacing or interlooping, but, in the case of non-woven yarn systems, are bonded together.
A triaxial woven fabric, FIG. 3, is a fabric made in a weaving process employing three yarns at 60.degree. angles to each other. The present inventors have found that the preferred triaxial fabric, especially if made of low denier yarns, is a thin fabric stable under various stresses even if the fabric is of high openness. Similarly, the reinforced membrane is stable during handling and installation, under the forces of shrinkage and expansion inside the electrolyzer, and during disassembly of the cell, allowing a higher percentage of the membrane to be reinstalled and reused. Furthermore, the dimensional stability of the membrane in all directions means that a piece for use in an electrolyzer can be cut to avoid a flaw in the roll of reinforced membrane, even if the piece is cut on the bias or at an acute angle to the machine direction. In the case of plain weave, the piece of membrane must be cut parallel or perpendicular to the machine direction in order to have dimensional stability in both the length and breadth of the piece.
Additionally, membrane reinforced with triaxial fabric is superior in resistance to "pin holes" or, in other words, tiny holes in the membrane which allow leakage of one electrolyte into the other electrolyte. When membranes of the prior art are pretreated with a dilute base or salt to expand them and convert them to the desired ionic form (usually the Na.sup.+ form) and then spread horizontally, while wet and limp, onto an electrode during assembly of the cell, tiny crimps may occur. These become tiny cracks that become pin holes. The membrane of the present invention is superior in this respect.